1. Field of Invention
The present invention relates to a time adjustment device that corrects the time based on signals from a positioning information satellite such as a GPS satellite, to a timepiece that has the time adjustment device, and to a time adjustment method.
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on known orbits, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the satellite time or satellite time information) with extremely high precision.
Electronic timepieces that adjust the time using this GPS satellite time information (satellite time information) are known from the literature.
In an electronic timepiece that uses GPS satellite time information (satellite time information), the reception unit that receives signals (navigation messages) from the GPS satellites must receive the TOW signal and the WN signal that are contained in the GPS satellite signals in order to get the time information from the GPS satellite. The TOW (Time of Week) signal is also referred to as the Z count and is a GPS time value that restarts every week and denotes the number of seconds from the beginning of the GPS week. The WN (Week Number) signal is the number of the GPS week in which the current GPS time is contained.
Power consumption increases greatly if the electronic timepiece constantly receives the GPS satellite signals.
Japanese Unexamined Patent Appl. Pub. JP-A-H10-82875 therefore teaches an electronic timepiece that switches between a full-power mode in which the CPU and other parts are driven to receive the GPS satellite signal and acquire the navigation message, and a sleep mode in which the CPU is stopped and the signal is not received, and thus reduces power consumption by intermittently receiving signals from the GPS satellites.
The electronic timepiece taught in JP-A-H10-82875 enters the full-power mode when the power is turned on and when the sleep mode ends to drive the CPU and other parts and acquire the navigation message. The timepiece then extracts the time information contained in the navigation message and adjusts the time.
The time when the next navigation message is acquired, or more specifically the time for correcting the time, is then determined from the relationship between the precision of the crystal (crystal oscillator) that produces the reference clock signal of the electronic timepiece and the required accuracy of the timepiece.
The timepiece then operates in the sleep mode during which the CPU is stopped until it is time to receive the navigation message again. Operation then returns to the full-power mode after this sleep mode period passes to acquire the next navigation message and adjust the time based on the time information extracted from the navigation message.
The electronic timepiece described above enters the full-power mode and acquires the navigation message after the power turns on and when the sleep mode ends. In the full-power mode, the electronic timepiece then acquires time-related data including the Z count, the week number, the UTC parameters, and the satellite signal correction amount.
The Z count can be received every 6 seconds in the GPS satellite signal, but because the week number and other information is carried in subframe 1, which is transmitted every 30 seconds, more time is required when receiving data other than the Z count than when acquiring only the Z count.
This means that each navigation message acquisition process executed in the full-power mode is time-consuming and increased power consumption becomes a problem. As a result, when reducing power consumption in order to increase battery life is desirable such as in a wristwatch, high power consumption in the full-power mode makes it difficult to achieve the desired effect even if power consumption is reduced by inserting a sleep mode.
If much time is needed to acquire the navigation message in a mobile electronic timepiece such as a wristwatch, there is a strong possibility that movement of the electronic timepiece while the signal is being received will change the orientation of the antenna or result in the GPS satellite being hidden by a building. This increases the likelihood of a navigation message acquisition failure, and a corresponding increase in power consumption.
One conceivable solution is to inform the user that the navigation message is being received and direct the user to not move the timepiece during this time, but the timepiece must remain stationary for a relatively long time and ease of use is thus impaired.